Methods for detecting combustion misfires in multi-cylinder internal combustion engines are known from the state of the art wherein combustion misfires are detected from data for the rough running of individual cylinders, which are determined in the engine, via a comparison to threshold values at operating points stored in a characteristic field. The threshold values are usually stored in a characteristic field for individual operating points of the engine. Combustion misfires below an rpm-dependent zero-load characteristic line are suppressed.
The methods are essentially based on detecting combustion misfires from values for the rough running of individual cylinders of the engine by a comparison to fixed threshold values in that the determined rough-running values exceed the threshold value. The values are determined internally in the engine.
The rough-running values for individual cylinders, which are decisive for the detection of misfires, are obtained while using the realization that a combustion within a cylinder of an engine, which does not take place or does take place only incompletely, is associated with characteristic changes of the torque trace of the engine compared to the normal operation. One can distinguish between normal operation of the engine without misfires and operation subjected to misfires from the comparison of the torque traces, that is, the amount which individual cylinders contribute to the torque. The differences in the torque trace are determined by determining segment times. Here, consideration is given to the situation that a torque contribution, which is lower as a consequence of an occurring misfire, is associated with a lengthening of the segment times of the crankshaft. The respective piston movement of individual cylinders during the torque producing expansion phase corresponds to a crankshaft segment. The rough-running values of individual cylinders are determined in a manner known per se from the measured segment times and are compared to the above-mentioned threshold values in a follow-on method step. Such methods for detecting combustion misfires have been proven in principle and are applied in the positive load range of the engine above a so-called zero-load characteristic line. Below the zero-load characteristic line, which is individually fixed for each vehicle engine, the combustion misfires are suppressed. In the determination of the decisive zero-load characteristic line, it should be considered that, for specific engine systems (for example, xcex=1 systems), the load data are dependent upon external conditions. The ambient pressure and therefore the counterpressure which opposes the exhaust gas are considered as external conditions.
Furthermore, it has been shown in practice that in the rpm region close to idle, the zero-load level can fluctuate slightly from vehicle to vehicle.
From these conditions, the problematic results that a detection of a combustion misfire in the region of the zero characteristic line is suppressed, under some circumstances, too early or too late.
In view of the problematic referred to above, the method of the invention affords the advantage that a precise zero-load characteristic line is determined for all operating states by adapting the zero-load characteristic line during vehicle operation continuously to the actually present lowest loads at corresponding rpm values so that the combustion misfire detection is always active in the statutorily prescribed regions and an incorrect detection because of a defective suppression cannot occur.
Because of the continuous monitoring of the zero-load level, vehicle specific differences as well as differences in ambient pressure can also be considered. The differences in ambient pressure can occur, for example, because of different levels of elevation.
A preferred configuration of the method of the invention provides that actual load values rl are measured for actual rpm points and are compared to the zero-load values rlK determined from the stored characteristic line. A dropping of the load values rl below the zero-load values rlK effects an assumption of the load values rl into the characteristic line values. In this way, and via simple structural measures within an engine management system, the zero-load characteristic line can be applied for the lowest load points. The zero-load values of the characteristic line can be adapted in the direction of the actually occurring low loads via corresponding filtering.
The region of the zero-load characteristic line is seldom touched at high rpms. For this reason, it has furthermore been shown to be advantageous that, for low rpms (preferably in the idle range) a difference xcex94rl=(rlxe2x88x92rlK) is determined from a comparison of the measured zero-load values rl with the zero-load values rlK determined from the characteristic line; and, from this difference xcex94rl, for higher rpms, a corrected zero-load value rlNEW=(rlK+xcex94rl) is determined which is taken into the zero-load characteristic line. In this way, the difference of the actual load values, which is present in idle, to the characteristic line load values is considered for the entire rpm range. However, a condition precedent for the application is that it must be made certain that additional consumers within a motor vehicle, which could lead to a higher idle rpm level, are disabled during the determination of the actual zero-load values rl. Furthermore, for specific application purposes, it can be advantageous to utilize the determined difference xcex94rl from the idle level only at a fixed percentage for the new determination of the zero-load characteristic line.